When tracking is lost, your avatar resets back to the default pose. Is there any way to stop this and have it idle in the last know pose until it tracks you again?

Hi there! It is not possible to change its idle position - at least in FaceRig. If you had a custom model with an idle pose in which the avatar is sitting in a different position than that resting pose, you could achieve this. However, in FaceRig, with the avatars that are available, this cannot be achieved.

So if the model has an idle pose, you can make it so that if tracking is lost then the avatar doesn't move until tracking is picked up again?

That is the behavior in FaceRig, yes. The avatar returns to its idle pose every time the tracking gets lost. I am not sure that I understood your question properly, though. The avatar won't move until the tracking gets picked up.